Recent years have seen an increase in applications that employ compressed images such as person monitoring in city centers, for instance, and video telephony. It is therein known that a significantly impaired image quality has to be accepted when image and video data is highly compressed. A widely applied known criterion for assessing the quality of image and video data is a PSNR (PSNR: Peak Signal-to-Noise Ratio) method. That entails summating differences in corresponding pixels of an image prior to encoding and of an image reconstructed based on the encoding and decoding, then representing the summated value as a logarithmic value. That, though, requires the pixels of the image data to be available prior to encoding.
A known method for rating the quality of reconstructed image data employs known compression artifacts. Instances thereof occurring in the case of, for instance, JPEG (JPEG: Joint Picture Expert Group) and MPEG-2 (MPEG: Motion Picture Expert Group) are disruptive block artifacts that can be quantified easily and relatively reliably by analyzing brightness steps at block boundaries. A mean change in brightness at the block boundaries then serves as a measure of the diminution in quality of compressed images; see [1], for example. That method fails, however, in the case of compression techniques where the brightness steps at the block boundaries or, as the case may be, block edges are smoothed. That is the case with, for example, an ITU-H.264 (ITU: International Telecommunication Union) video-compression standard because H.264 employs an adaptive “deblocking filter” for eliminating brightness steps at block boundaries.
A further known possibility for determining quality in reconstructed image and video data is to ascertain image blurring with the aid of edge analyses, as mentioned by way of example in document [2]. Blurring of such kind arises on the one hand from a lack of higher-frequency transform coefficients due to coarse quantizing. Image blurring is on the other hand further intensified by the deblocking filter in the case of the H.264 standard. However, the edge analyses often result in misclassification in the case of video and image signals such as those, for example, coded using the H.264 standard. High-contrast edges are observed to be retained even when compression is very high. The reasons are that the corresponding transform coefficients are not eliminated owing to the respectively high amplitude and that the deblocking filter is deactivated when the contrast is high.